Magnetostrictive torque sensor

ABSTRACT

A noncontacting method for sensing torque based on the principle of magnetostriction comprises inducing a primary magnetic flux in a shaft (20) via such means as a primary excitation core/coil(30/32), keeping primary path flux amplitude constant via such means as an auxiliary core/coil (48/50) and appropriate feedback circuitry, and obtaining a torque dependent signal which is a function of a secondary flux via such means as a secondary core/coil(34/36). Signal dependence on RPM, temperature, and material property inhomogeneities is eliminated or minimized, instantaneous torque measurement is possible, and mass production is made feasible. A second embodiment utilizes modified signal processing circuitry to eliminate spurious signal components. A third embodiment comprises a single core/coil(58/60) placed close to a shaft (20) which detects torque by means of voltage or current changes in the coil (60). Embodiment four comprises two single core/coils(58/60 and 59/61) such as those of embodiment three and signal processing circuitry which produces a signal minimally affected by RPM, temperature, and material property inhomogeneities. A fifth embodiment employs a plurality of sensors (66A/66B) strategically located around the shaft (20) to eliminate spurious signals which are due to bending stress and shaft misalignment.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a torque sensor based on the principle ofmagnetostriction, and more particularly, to an improved magnetostrictivetorque sensor which is simpler, more accurate, and more economical thanstate of the art sensors as well as more suitable to mass production andusage.

2. Description of Prior Art

Engineers and scientists have sought a simple, reliable, accurate meansfor measuring torque in rotating shafts for well over a century.Applications for such a torque measuring apparatus include diagnosis,prognosis, and load level monitoring of a vast number of different typesof rotary drive mechanisms such as automotive, ship, and plane engines;motors and generators of all types; oil drilling rigs; rotatingmachining tools; all electric power steering; robotics; and much more.

Further, measurement of mechanical power produced by an engine (or usedby a generator) cannot be made without knowing both torque androtational speed of the shaft. Hence there has heretofore been no readymeans to determine on-line power and efficiency of rotary drive devicessimply, accurately, and reliably. This has proven to be problematic inmany areas of modern technology, but it has been particularlytroublesome in attempts to develop modern automotive engine controlsystems which would improve fuel efficiency and optimize engineperformance.

Heretofore several methods have been developed for measuring torque inrotating shafts (see below), but none has been ideal. That is, no singlepresently known method offers all of the following desirable properties.

1. Contact free (no slip rings, etc.)

2. Reliable (low failure rate)

3. Accurate

4. Small and unobtrusive (requiring little shaft/engine re-work)

5. Inexpensive

6. Applicable at high as well as low speeds

7. Instantaneous torque measurement (not merely mean torque over severalrevolutions)

8. Amenable to mass production (not restricted to special testapparatus)

There are presently only four distinct methods for measuring torquedirectly in a rotating shaft. They are:

1. Twist angle of shaft measurement

2. Strain gauge sensor

3. Reaction force measurement

4. Magnetostrictive sensors

The twist angle method involves measurement of the angle of twist of ashaft and correlates this, using the material and dimensionalcharacteristics of the shaft, to torque. It entails a complicated andcumbersome mechanism with low sensitivity, calibration difficulties, andthe necessity of using two different locations along the shaft. Itinvariably entails extensive engine modification, a costly endeavor. Thestrain gauge approach requires bonding of strain gauges to the shaftsurface and relating strain measurement to torque. It is limited to lowspeed, is not amenable to mass production, lacks durability, and needssome means such as slip rings and brushes to bring the signal off of theshaft.

Reaction force measurement utilizes Newton's second law for rotationalmotion to relate force and motion of the engine mounts to shaft torque.The method must employ a large structure, has low sensitivity, is notfeasible for production runs, and measures driveline, not engine,torque.

Magnetostrictive torque sensors take advantage of the magnetostrictiveproperty of ferromagnetic materials whereby tension stress increases(and compressive stress decreases) a given magnetic induction field(i.e., the "B" field) carried by the material. A coil of wire ofarbitrary number of turns wrapped around an iron core is placed close tothe shaft and an electric current passing through the wire causes amagnetic field to be induced in the rotating shaft. In magnetostrictivesensor designs such as those described in U.S. Pat. Nos. 2,912,642 and4,589,290 a second coil of arbitrary number of turns wrapped around asecond iron core is then placed close to the shaft and used to measurethe change in the induction (the B field) which results from theincreased surface stress caused by the applied torque.

The magnetostrictive method has several advantages over the other threemethods, including

(1) Non-contact: no slip rings

(2) Not restricted to low speeds

(3) Measures torque of engine directly

(4) High sensitivity

(5) Economical

(6) Simple structure: no strain gauges, no large apparata

(7) Only one location anywhere on shaft: little engine rework

(8) Durable and reliable: no moving parts to cause mechanical failure,resistant to high pressure and temperature of engine environment

(9) Readily miniaturized: can be made unobtrusive

However, magnetostrictive torque sensors have heretofore been plaguedwith several major problems which have prevented them from becoming thestandard in the field. These are

(1) Output signal varies with RPM even at constant torque.

(2) Output signal varies with temperature.

(3) Spurious signal variation within one mechanical cycle (onerevolution of shaft) prohibits accurate instantaneous measurement oftorque: only average values over several shaft revolutions are possible.

(4) Correction methodologies such as those described in U.S. Pat. Nos.4,589,290 and 4,697,459 and SAE paper #870472 heretofore employed forproblems (1) to (3) above have not been able to reduce inaccuracies toan acceptable level.

(5) All such correction methodologies developed to date involvecomplicated and extensive electronic circuitry and/or additional sensorsfor temperature and RPM.

(6) Additionally, all such correction methodologies utilized to date areaffected by subtle individual shaft material and property variationssuch as residual stress, slight inhomogeneity in shaft magneticproperties, shaft tolerances/misalignment, and shaft bending stress.Hence such methodologies must be tailored specifically for eachindividual shaft and therefore are not suitable for mass production.

(7) Furthermore, such correction methodologies have to be re-calibratedrepeatedly over the lifetime of the shaft, since residual stress values,tolerances, misalignments, bending stresses, and even magnetic propertyinhomogeneities change over time (particularly in high temperatureenvironments such as those of automobile engines). Recalibration formechanisms such as automobile engines is so difficult as to render suchcorrection methodologies impractical.

OBJECTS AND ADVANTAGES OF THIS INVENTION

Accordingly, several general objects and advantages of thismagnetostrictive torque sensor invention over prior art are considerableimprovement in accuracy to well within acceptable levels, simplicity ofdesign, lowered cost, suitability for mass production, and practicalityfor continued usage. Such general objects and advantages are achieved bythe following specific objects and advantages.

(1) Elimination of signal dependence on shaft rotation speed.

(2) Elimination of signal dependence on temperature.

(3) Elimination of signal variations within a single shaft revolutiondue to shaft magnetic property inhomogeneities and residual stresses.

(4) Elimination of signal variations due to bending stress, shaftmisalignment, and tolerance variations.

(5) Instantaneous measurement of torque resulting from advantages (3)and (4) above.

(6) Elimination of signal dependence on individual shaft properties andhence suitability for mass production.

(7) Elimination of need to recalibrate sensing device during course ofshaft lifetime.

(8) Simple, effective signal processing circuitry and sensor orientationpermitting advantages (1) through (7) above.

Further objects and advantages of the invention will become apparentfrom a consideration of the drawings and ensuing description of it.

SUMMARY OF THE INVENTION

This magnetostrictive torque sensor invention solves essentially all ofthe problems associated with prior art through heretofore unrealizedinsight into the fundamental electromechanical principles underlyingprevious experimental results and by using that insight to devise asuperior design solution.

In prior art a coil of wire with current flowing through it induces aprimary magnetic induction field (B field) in a ferromagnetic shaftwhich is located close to the coil. If the shaft is not moving, aconstant amplitude AC current in the wire will produce a constantamplitude magnetic flux in the shaft where the flux amplitude depends onthe magnetic permeability of the shaft material. As torque is applied tothe shaft, the shaft experiences torsional stress as a result of theapplied torque. Through the phenomenon of magnetostriction, a secondarymagnetic induction field component arises at an angle to the primarymagnetic field direction and the magnitude of that second magnetic fieldflux is dependent on the amount of torque applied to the shaft. Thesecondary flux is measured via Faraday's law by placing a second coilclose to the shaft aligned in the direction of the secondary magneticfield and then measuring the voltage across the second coil. The voltageacross the second coil is therefore a direct indicator of torque. Theprocedure is accurate if the primary field flux can be maintainedconstant.

If the shaft is turning, however, and if the shaft has local variationsin permeability around its circumference (true of virtually all shafts),then the flux amplitude in the shaft is not constant and varies as theshaft turns.

A different shaft, with different permeability, will also result indifferent primary magnetic flux, yielding a different voltage across thesecondary coil, and hence prohibiting use of the sensor on a massproduction scale.

This problem is solved in the present invention by using an auxiliarycoil (i.e., a third coil) to measure the primary magnetic flux viaFaraday's law. Feedback of the signal from the auxiliary coil to thepower supply for the primary coil is done in such a manner as tomaintain the auxiliary coil voltage amplitude constant, therebymaintaining the primary flux amplitude constant regardless of changes inshaft permeability due to rotation of a single shaft or use of differentshafts. Hence the torque indicating output signal from the secondvoltage will be independent of shaft speed or shaft materialcomposition.

A second embodiment of the invention divides the secondary coil voltageby the auxiliary coil voltage to obtain a signal which has a similarintegrity. The second coil voltage varies with RPM, shaft material, andtorque whereas the auxiliary coil voltage varies with RPM and shaftmaterial, but not torque. Hence the resultant signal after division isfree of spurious signal components and is an accurate indication oftorque.

A third embodiment utilizes a single coil at constant amplitude ACcurrent to induce flux in a shaft. As torque is applied to the shaft,due to magnetostriction the voltage in the coil must change to maintainconstant current amplitude in the coil. The voltage signal is a functionof torque applied and hence it or a function of it is used to indicatetorque. Alternatively, constant voltage can be used in the coil andcurrent change related to torque change.

Embodiment four uses two sensors such as those in embodiment three toeliminate spurious signal components from RPM and shaft variation.

Embodiment five employs two or more magnetostrictive torque sensors ofany of embodiments one through four by placing the sensors strategicallyaround the shaft such that a resultant signal produced by combining thesignals of the individual sensors is free of erroneous signal componentsresulting from bending or misalignment of the shaft.

The invention, in its various embodiments, solves each of the problemsassociated with prior art discussed in the "Description of Prior Art"section in a superior and wholly satisfactory manner.

BRIEF DESCRIPTION OF THE DRAWINGS

Drawings Illustrating Prior Art

FIGS. 1A and 1B depict the behavior of an induced magnetic field B in ashaft undergoing applied torque. FIG. 1A depicts the zero torque state;FIG. 1B the state with applied torque.

FIGS. 2A and 2B show the standard prior art configurationmagnetostrictive torque sensor. FIG. 2A depicts the physical arrangementof the primary and secondary coil/cores; FIG. 2B depicts the signalprocessor circuit block diagram.

Drawings Illustrating Problems and Limitations of Prior Art

FIG. 3 is a reproduction from SAE paper #870472 showing the signalvariation in output signal with constant torque over one mechanicalcycle which is typical of prior art configurations.

FIG. 4 is also a reproduction from SAE paper #870472 and shows thevariation in output signal with shaft rotation speed which is found inprior art configurations.

Drawings Illustrating the Present Invention

FIG. 5A depicts the signal processor block diagram representing onevariation of embodiment number one of the present invention. FIG. 5Bshows a second variation of embodiment number one.

FIG. 6A depicts the signal processor block diagram representing onevariation of embodiment number two of the present invention. FIG. 6Bshows a second variation of embodiment number two.

FIG. 7A depicts the single coil/core and signal processor block diagramsof one variation of embodiment number three of the present invention.FIG. 7B shows a second variation of embodiment number three.

FIGS. 8A and 8B depict embodiment number four of the present invention.FIG. 8A shows two single coil/core sensors such as those of embodimentthree whose coil output voltages are combined via the signal processorblock diagram of FIG. 8B.

FIG. 9 depicts embodiment number five of the present invention,comprising a multiplicity of magnetostrictive torque sensorsstrategically located such that the sum of the voltages produced by eachsensor is a signal which is effectively free of misalignment and bendingstress induced signals.

FIG. 10 depicts a variation of embodiment number five, in which a pairof magnetostrictive torque sensors have unequal angular spacingtherebetween.

DRAWING REFERENCE NUMBERS

20 rotating shaft

30 primary core

32 primary coil

34 secondary (pickup) core

36 secondary (pickup) coil

38 voltmeter measuring output of secondary coil

40 oscillator

42 power amplifier

44 amp meter

46 low pass filter

48 auxiliary coil (on primary core)

50 auxiliary coil voltmeter

52 auxiliary coil (on primary core) for embodiment number two

54 auxiliary coil voltmeter for embodiment number two

56 signal divider

58 core of embodiment three and four

59 second core of embodiment four

60 coil of embodiment three and four

61 second coil of embodiment four

62 voltmeter of embodiment three

66A magnetostrictive sensor A

66B magnetostrictive sensor B

68A air gap between sensor A (item 66A) and shaft (item 20)

68B air gap between sensor B (item 66B) and shaft (item 20)

70c bending compressive stress

70t bending tensile stress

PRINCIPLES AND ANALYSIS OF MAGNETOSTRICTIVE TORQUE SENSORS

Conceptual Overview

FIGS. 1A and 1B show the effect on an induced magnetic field (alsocalled induction) B in a shaft 20 when torque is applied to the shaft20. FIG. 1A depicts the shaft 20 with zero torque applied and theinduction B directed circumferentially. The induction B can also berepresented in terms of two components B_(x) and B_(y) at 45° angles tothe circumferential direction.

FIG. 1B illustrates the change in the induction B when torque isapplied. The torque creates tensile stress σ_(t) in the direction of theinduction component B_(x), and compressive stress σ_(c) in the directionof the induction component B_(y). Due to the principle ofmagnetostriction, induction component B_(x) increases in magnitude toB_(x) ', and induction component B_(y) decreases in magnitude to B_(y)'. The induction components and can then be added vectorially to obtainthe induction with torque applied B'. The induction B' makes an angle δwith respect to the original induction B, and can then be represented interms of two components B_(circ) ' and B_(axial) ' directed along thecircumferential and axial directions respectively. Therefore,

    B.sub.circ '=B' cos δ

    B.sub.axial '=B' sin δ.

The angle δ is in practice quite small, hence

    B'.sub.circ ≈B'≈B

    B'.sub.axial ≈B'δ≈Bδ.

The magnitude of B_(axial) ' increases with increasing applied torqueand equals zero when torque equals zero.

FIGS. 2A and 2B depict the standard prior art configuration for inducingthe magnetic induction B and for measuring the induced axial inductionB_(axial) '. FIG. 2A shows the primary coil 32 carrying current,typically AC, wrapped around the primary ferromagnetic U shaped core 30in which the core 30 is aligned circumferentially to shaft 20 and inwhich the ends of the core 30 are shaped and placed such that a constantwidth air gap exists between the core 30 and the shaft 20. The currentin primary coil 32 causes a magnetic flux to be carried through primarycore 30 across the air gaps and along the surface of shaft 20 in amanner similar to that depicted in FIGS. 1A and 1B.

Secondary coil 36 is typically open circuit and is wound aroundsecondary U shaped core 34 which is aligned in the axial direction ofshaft 20 and has ends shaped and oriented such that constant width airgaps exist between the secondary core 34 and the shaft 20. Applicationof torque to the shaft when current is passing through the primary coil32 causes axial induction B_(axial) ' to arise and magnetic flux to passthrough secondary core 34. Voltage is then produced in secondary coil 36via Faraday's law and measured by voltmeter 38. This voltage is zero forzero applied torque and increases as torque increases; hence, thevoltage is a direct measure of applied torque.

Limitations of Idealized Approach

FIG. 3 is a reproduction from SAE paper #870472 showing the spuriousoutput signal variation measured by voltmeter 38 (see FIG. 2B) withineach single revolution (mechanical cycle) of shaft 20.

This phenomenon is attributed to local variations in magneticpermeability and residual stress around the shaft circumference. Theshaft material is not completely homogeneous either in its magneticproperties or in degree and distribution of residual stresses. As theshaft rotates, these non-uniformities pass under the sensor and alterthe amount of flux transmitted through the sensor coils. The result is anon-torque dependent variation in the output voltage.

A second cause for spurious subcycle signal alteration is variation inair gap thickness between sensor and shaft due to (i) shaft misalignmentand/or (ii) bending stresses in the shaft. Since the flux is alsodependent on this gap thickness, if the gap thickness varies then sowill the output voltage.

The standard solution to this problem has been to use a low pass filterto eliminate all signal variations with frequencies greater than that ofone shaft revolution (one mechanical cycle.) Unfortunately, thisintroduces a time constant into the device which limits the measurementbandwidth to approximately 1/2 the mechanical frequency and precludesmeasurement of torque changes more rapid than those occurring overseveral mechanical cycles. The resulting signal becomes an averagerather than an instantaneous measure of torque and is hence limited tosteady state or slow transient operations.

FIG. 4 is also a reproduction from SAE paper #870472 and illustrates asecond problem encountered in prior art-the variation in output signalwith RPM. Increasing the speed of rotation of the shaft 20 while torqueremains constant increases the output signal. This is undesirable sincethe ideal signal should reflect change in torque alone. The commonsolution approach heretofore employed has been to monitor RPM andintroduce a shaft speed dependent correction to the output signaldetected by voltmeter 38. This has not been completely successful,eliminating some but not all of the error. Further, it has complicatedthe device by introducing additional measurement and apparata.

A third problem in prior art is temperature dependence. As the shafttemperature varies (a common phenomenon in automobile and many otherengines), so does the output signal even as all other parameters remainconstant. This is no doubt due in large part to (i) temperature inducedvariations in magnetic permeabilities of the shaft/sensor materials,(ii) concomitant modification in tolerances which affect the air gapdimension between sensor and shaft, and (iii) changes in resistivity ofthe wires used in the sensor. The typical prior art solution is toemploy feedback from a thermocouple to further correct the outputsignal. As reported in SAE paper #870472 this also has had only some butnot full success. This is presumably because different temperaturegradation fields within the engine would affect the sensor/shaftdifferently yet could have the same temperature value for any givensingle point (i.e., the point at which the thermocouple might belocated.)

Signal instability or drift over time is yet a further problem. Duringtesting, the SAE paper #870472 researchers found it necessary to "nullout" the zero torque signal anew each day.

In summary, several disadvantages of prior art magnetostrictive torquesensors are:

(a) Signal variation within one mechanical cycle prohibits accurateinstantaneous torque measurement.

Average values over several cycles must be used. Causes are:

(i) Local random variations in magnetic material properties

(ii) Sinusoidal variation in air gap due to

shaft misalignment

bending stresses in shaft

(b) Signal drifts from day to day

(c) Output varies with RPM

(d) Output varies with temperature

Analysis of Prior Art Limitations

Faraday's law of electromagnetism, i.e., ##EQU1## where φ is magneticflux and A is core cross sectional area, can be rewritten using thedefinition for inductance L of a coil/core, i.e.,

    Nφ=Li                                                  (II)

where N is number of coil windings and i is current in the coil.

Hence Faraday's law can be rewritten as ##EQU2##

In virtually all electric circuit applications L is constant and thesecond term in the far right hand side of (III) above vanishes. In thepresent case, however, it does not vanish, and as is shown below, thesecond term gives rise to the phenomenon of output signal dependence onRPM depicted in FIG. 4.

Inductance L depends on magnetic permeability μ_(Fe) of the shaft, andpermeability varies slightly in different places around the shaftcircumference. This variation can be due to either (i) natural changesin the material grain structure, composition, etc. or (ii) residualstresses on the shaft surface from machining, forming, etc. (whichchange μ_(Fe) via magnetostriction.) Hence, as the shaft rotates, thesensor "sees" a varying inductance through which its flux must pass. Thetime derivative of L is therefore not zero and the second term in thefar right hand side of (III) above becomes important, i.e., it makes acontribution to the output voltage. Further, since the time derivativereflects the rate of change of L, the time derivative increases inmagnitude for increasing values of RPM. Hence, the RMS value of thesecond term in the right hand side of (III) increases with increasingrotational speed of the shaft, whereas the RMS value of the first termalone does not.

It is the second term which is responsible for the heretoforeunexplained variation in signal output with shaft RPM. In addition, asdiscussed previously, the second term is also partly responsible for theirregular variation in signal within one shaft revolution shown in FIG.3.

Additionally, it is the second term which causes each individual shaftto have a different dependence on RPM, since each shaft has differentlocal variations in residual stress and magnetic permeability. Hencecorrection methodologies such as those heretofore employed which attemptto correct for output signal dependence via preset feedback signalcorrection are not suitable for mass production. Further, the correctionneeded for each individual shaft will change with time as residualstresses relax and as temperature and stress alterations modify localpermeability variations. Hence the heretofore used correctionmethodologies would become increasingly inaccurate over time and wouldneed continual recalibration. This is impractical in most applications,but particularly so for automotive and other vehicle engines.

Conclusion: Local variations in permeability and residual stressesresult in:

(a) Signal irregularities within one shaft revolution which must be"averaged out" over several cycles

(b) Increase in output signal with RPM.

Correction methodologies used heretofore are unsuitable for massproduction and become increasingly inaccurate over time.

DETAILED DESCRIPTION OF THE INVENTION

Previous art has maintained a long held tradition in electromechanics.That is, it has used an electronic feedback loop to keep input currenti_(p) to primary coil 32 at a constant RMS value. The objective in doingthis has been to keep primary core 30 flux φ_(p) at constant RMS (see(II).) The underlying assumption is that L, the inductance, remainsconstant. In the case of magnetostrictive torque sensors, however, theinductance L does not remain constant, so flux φ_(p) varies even ifi_(p) remains fixed. This results in the spurious output voltage signaldiscussed in previous sections. The present invention, in differentembodiments, involves alternative methodologies which solve this andother problems associated with prior art.

Embodiment 1: Feedback to keep φ_(p) RMS constant

FIG. 5A depicts circuitry to be used with the magnetostrictive torquesensing apparatus of FIG. 2A in lieu of the signal processor of FIG. 2B.The volt meter 50 measures open circuit voltage in an auxiliary coil 48of any number of turns which in addition to primary coil 32 is alsowrapped around the primary core 30. From Faraday's Law (I) the voltmeter50 can be used to measure flux φ_(p) in the primary core 30. Flux φ_(p)in the primary coil 32 can be monitored and used in a feedback to thepower amplifier 42 which continuously adjusts input voltage v_(in) suchthat φ_(p) rather than i_(p), is kept at constant RMS value. Keeping thevoltage amplitude of voltmeter 50 constant (by varying v_(in)) keeps theamplitude of primary flux φ_(p) constant. In this way local variationsin inductance L around the circumference of the shaft do not causevariations in the primary flux φ_(p). The result is a circumferentialinduction B_(circ) and an axial induction B_(axial) which are virtuallyfree of anomalies caused by surface inhomogeneities. Hence the outputsignal (from voltmeter 38) is essentially independent of RPM andconsiderably more accurate than existing devices. The output signal fromvoltmeter 38 is also of relatively constant amplitude within onemechanical cycle (for constant torque within one mechanical cycle) andtherefore is suitable for detecting variation in torque virtuallyinstantaneously.

In some instances this embodiment has an even simpler modification asshown in FIG. 5B. For certain values of circuit parameters the feedbackof the signal from voltmeter 38 does not have to be done. Note from (I)that time varying flux in the primary core 30 means voltage drop acrossthe primary coil 32. There is internal resistance in the primary coil 32as well, so the driving voltage v_(in) must equal the total of theinductive voltage -v_(coil) produced by time varying flux φ_(p) of theprimary coil 32 plus the voltage drop across the internal resistanceR_(p) of the primary coil circuit. That is ##EQU3##

When resistance R_(p) in the primary coil 32 is negligible (such asmight be encountered with use of superconductor materials or with fewwindings in the primary coil 32), the second term in the right hand sideof (IV) can be assumed effectively zero. In that case the input voltage(v_(in)) is equal and opposite to the primary coil voltage -v_(coil)associated with the oscillating primary flux φ_(p). Hence keeping inputvoltage v_(in) (rather than input current i_(p)) at constant amplitudekeeps primary flux amplitude φ_(p) constant regardless of anynon-uniformity in shaft magnetic properties.

Note that in this or any other embodiment the primary core/coil 30/32can be aligned axially instead of circumferentially and the secondarycore/coil 34/36 can be aligned circumferentially instead of axially.Further, even though the device performs optimally when the secondaryand primary core/coils 34/36 and 30/32 are at right angles to oneanother and when one of the coil/cores is axially aligned, they can infact be at any angle with respect to one another and with respect to theshaft axis.

The auxiliary coil 48 is depicted as wrapped around primary core 30 andconcentric with primary coil 32, but all that is essential to the properworking of embodiment one is that a significant portion of the flux inthe primary flux path of the shaft 20 pass through auxiliary coil 48.Hence auxiliary coil 48 can be wrapped inside of, wrapped outside of,wound along with, located elsewhere on primary core 30 than, or placedin close proximity to primary coil 32; or placed anywhere else so longas the signal obtained from auxiliary coil 48 or a function of thatsignal can be used to control the primary flux in the shaft 20.

The description of embodiment one contains many specificities, butembodiment one is not limited in scope by these specificities.Embodiment one is primarily a method to keep primary flux at effectivelyconstant amplitude, thereby eliminating spurious signal components dueto material inhomogeneities, temperature, and shaft speed. Embodimentone, therefore, encompasses any method which maintains primary flux atsuch an effectively constant amplitude and monitors torque viadetermination of secondary flux.

Embodiment 2: Divide Output Signal by Input Voltage

FIGS. 6A and 6B depict different variations of embodiment number two ofthe present invention. In embodiment number two the standard prior artapproach is used for the primary circuit in which primary current i_(p)amplitude is kept constant. For the constant primary current i_(p)amplitude local variation in magnetic permeability μ_(Fe) will bereflected in both the B_(circ) and the B_(axial) fields. Output voltagefrom secondary coil 36 depends on the time derivative of B_(axial) (see(I).)

In FIG. 6A an auxiliary coil 52 around the primary core produces avoltage signal (measured by voltmeter 54) dependent on the timederivative of the primary B_(p) field (i.e., on φ_(p). Both signalsincorporate the local inhomogeneities, but only the output of thesecondary includes changes due to torque induced stresses. Dividing theinstantaneous secondary voltage produced by secondary coil 36 by theinstantaneous auxiliary coil voltage measured by voltmeter 54 by meansof signal divider 56 results in a signal measured by voltmeter 38 whichis essentially free of inhomogeneity induced variation. The signalmeasured by voltmeter 38 nevertheless still depends directly on theapplied stress in the shaft and is a good measure of instantaneoustorque.

In practical application it may at times prove necessary to add acomponent to the circuit between voltmeter 54 and signal divider 56which would convert instantaneous zero signal values (measured byvoltmeter 54) to small finite values in order to preclude division byzero in signal divider 56.

As with embodiment #1, negligible resistance in primary coil 32 wouldresult in the simplification of embodiment #2 shown in FIG. 6B. Withnegligible primary coil 32 resistance R_(p), voltage in the primary coil32 itself can be used directly to divide into the voltage from thesecondary coil 36, thereby eliminating the need for auxiliary coil 52 onthe primary core.

As with embodiment one, the primary core/coil 30/32 can be alignedaxially instead of circumferentially and the secondary core/coil 34/36can be aligned circumferentially instead of axially. Further, eventhough the device performs optimally when the secondary and primarycore/coils 34/36 and 30/32 are at right angles to one another and whenone of the coil/cores is axially aligned, they can in fact be at anyangle with respect to one another and with respect to the shaft axis.

The auxiliary coil 52 is depicted as wrapped around primary core 30 andconcentric with primary coil 32, but all that is essential to the properworking of embodiment two is that a significant portion of the flux inthe primary flux path of the shaft 20 pass through auxiliary coil 52.Hence auxiliary coil 52 can be wrapped inside of, wrapped outside of,wound along with, located elsewhere on primary core 30 than, or placedin close proximity to primary coil 32; or located anywhere else so longas the signal obtained from auxiliary coil 52 or a function of thatsignal can be used as a meaningful input to signal divider 56.

Although FIGS. 6A and 6B and the above discussion relates to a divisionof the signal produced by secondary coil 36 by either the signalproduced by auxiliary coil 52 or primary coil 32, embodiment two equallyrelates to a division of the signal produced by auxiliary coil 52 orprimary coil 32 by the signal produced by the secondary coil 36.Further, the signals divided can be instantaneous, RMS, amplitude or anyother indicator of signal strength.

The description of embodiment two contains many specificities, butembodiment two is not limited in scope by the specificities. Embodimenttwo is primarily a method to eliminate spurious signal components due tomaterial inhomogeneities, temperature, and shaft speed by dividing twosignals each of which contain similar such spurious signal components,but only one of which contains a torque dependent component. Embodimenttwo, therefore, encompasses any method which accomplishes such adivision.

Concomitant Solutions to Variation in RPM, Temperature, Drift Problems

Spurious material non-uniformity induced frequency components of theoutput voltage signal from secondary coil 36 are directly dependent onshaft speed. Via embodiment #1 or #2, the spurious frequency componentsare eliminated from the output signal measured by voltmeter 38 anddependence of the output signal of voltmeter 38 on shaft speed isminimized and reduced to an inconsequential level.

Temperature dependence in both embodiments is minimized as well. Inembodiment #1 variations due to temperature dependent permeability andmechanical tolerances will automatically be compensated for by keepingthe primary flux φ_(p) amplitude constant. In addition, thermalvariations in wire resistivity would not give rise to differences insecondary voltage measured by voltmeter 38 since the second circuit isopen circuit.

In embodiment #2, both voltage signals entering signal divider 56 areaffected by temperature and tolerance variation in the same way andhence when the signals are divided, variation caused by temperature andtolerances drops out of the resulting signal measured by voltmeter 38.

Although drift in signal is not a serious problem since it can always be"nulled out" prior to measurement, it should be noted that drift,particularly in embodiment two is reduced as well. Drift affects bothsignals entering signal divider 56 in a similar way, and hence, whenthey are divided, effects from drift are minimized.

Embodiment 3: A Single Coil/Core to Measure Torque

FIGS. 7A and 7B illustrate embodiment three in which a single coil ofany number of turns is used to measure torque applied to a rotating orstationary shaft.

The core 58 and coil 60 of embodiment three are aligned along adirection of principle stress in shaft 20, i.e., at a 45° angle to themain shaft axis. In FIG. 7A feedback from ammeter 44 to power amplifier42 maintains constant amplitude current into coil 60. As torque isapplied to shaft 20 stress on the shaft surface changes the magnitude ofthe induction field B induced on the shaft surface by the core/coil58/60. For constant current amplitude the voltage across coil 60measured by voltmeter 62 must then change and the change in voltage mustbe directly related to intensity of applied torque. With propercalibration, the readout of voltmeter 62 is a direct measure of appliedtorque.

FIG. 7B is an alternative form of embodiment three in which inputvoltage from power amplifier 42 is kept at constant amplitude andcurrent is allowed to vary. As torque is applied to the shaft, the fluxpassing through coil 58 changes and hence so does the current passingthrough the coil. The readout from ammeter 44 then varies directly withapplied torque and can be used to measure torque applied to the shaft.

The coil/core 58/60 of embodiment three ideally can be aligned alongeither of the directions coincident with the principle stresses.Alternatively the coil/core could be aligned in any direction relativeto the shaft axis and a meaningful (though not necessarily optimum)signal could be obtained.

Although embodiment three is described herein with reference to voltageacross and current in the coil, those skilled in the art will recognizethat other circuit parameters, such as voltage or current in other partsof the circuit, are also dependent on magnetostrictively producedchanges in permeability of the shaft and hence can be used to determinetorque. Embodiment three therefore also relates to uses of other suchcircuit parameters to determine torque.

Embodiment 4: Duel Single Coil/Core Apparatus

FIG. 8A depicts embodiment four comprising an enhancement of the singlecoil/core device of FIGS. 7A and 7B whereby a second single coil/core59/61 is aligned at right angles to single core/coil 58/60, i.e., alongthe direction of the second principle stress (in the ideal case). Ameaningful measure of torque would then be the signal resulting fromsubtraction of the voltage signal from one of the single coils from thevoltage signal of the other single coil, e.g., v_(t) -v_(c). If thissignal were free of inhomogeneity caused spurious signals it could beused directly as an effective indicator of torque. If such inhomogeneitycaused spurious signals were present, however, this signal could bedivided by v_(t) +v_(c) to obtain a resultant signal v_(T) which wouldbe effectively free of such spurious signals. A signal processor blockdiagram accomplishing this is depicted in FIG. 8B.

One major advantage of the enhancement is that zero output signal v_(T)would correspond to zero torque and hence, for example, a twofoldincrease in torque would result in a twofold increase in output signalv_(T). In the single coil version on the other hand, a large nonzerooutput signal would be measured by voltmeter 62 even when no torque isapplied. Doubling of torque from any given value would result in only asmall pecentage increase in output signal and hence accuracy in thesingle coil version would be markedly worse.

Although the discussion of embodiment 4 depicted in FIGS. 8A and 8Brelates to changes in voltage for constant amplitude current, embodiment4 equally as well relates to changes in current for constant amplitudevoltage. Further, the single core/coil 58/60 can be at any angle and thesecondary core/coil 59/61 can be at any nonzero angle with respect tosingle core/coil 58/60.

Embodiment 5: Multiple Sensors to Solve Misalignment, Bending StressProblems

FIG. 9 illustrates a modification which eliminates spurious outputsignal caused by shaft bending stresses and/or shaft misalignment andwhich is applicable to any of embodiments one through four as well as toprior art.

The inductance L is dependent on the air gap dimension. For shaftdisplacement from centerline due to slight misalignment or bending, theinductance L will change and hence so will the output signal (even forthe embodiments presented above.) In addition, stresses from bendingwill affect permeability (magnetostriction again) and further alter theoutput. Both of these effects contribute erroneous components to theoutput signal which are sinusoidal with period equal to the time of oneshaft revolution.

This problem can be ameliorated in either of two ways:

(1) Place sensory in an end or main bearing where no bending stressexists and misalignment can be minimized. (Note that though this is notspecifically claimed as part of the invention described herein, theinvention and any of its embodiments described herein can be used in anylocation along a shaft including the end or main bearing. The readerwill understand that a specific location cannot be claimed as a patentright.)

(2) Use two sensors 66A and 66B on opposite sides of the shaft as inFIG. 9 and add the two (instantaneous) signals v_(A) and v_(B). Forsimplicity FIG. 9 is shown with zero applied torque although the methodis applicable with torque of any magnitude.

Method (2) above works because the misalignment and bending stresssignal aberrations are 180° out of phase and so cancel one another whenadded. This second method has the added advantage of doubling effectivesignal strength (i.e., the sensitivity).

Method (2) functions optimally when sensors 66A and 66B are on oppositesides of the shaft. The sensors can, however, have any angular relationto one another as long as proper phase corrections are made to one orboth signals to correct for the variation from 180° angular spacingbetween sensors (see FIG. 10). Further, embodiment five relates to anyorientation of the sensors which is sufficiently close to 180° angularseparation such that the sensor signals can be added directly withoutphase shifting to achieve a sufficiently accurate signal.

While the discussion of embodiment five relates to a single pair ofsensors, any number of pairs may be employed. Alternatively, a group ofmore than two sensors could be used in which signals from all sensorsare added to obtain a resultant signal free of misalignment and bendingstress induced components. For example, signals from three sensorsspaced 120° apart, or sufficiently close to 120°, could be directlysummed. For spacing other than 120°, signal phase shifting can be donewhere necessary before summation.

Solutions to Mass Production and Recalibration Problems

The reader will note that the invention solves the problems of RPMdependence, temperature dependence, and non-instantaneous torquemeasurement in a simpler, more elegant, and more effective manner thanto prior art device. At the same time the present invention solves theheretofore intractable problems of impracticability for mass productionand need for periodic recalibration during the lifetime of the shaft. Itdoes this because unlike prior art, the correction methodologies of thisinvention are independent of the particular properties (magnetic,residual stress, tolerance, and misalignment) of each particular shaft.In embodiment one, for example, the primary induction B and hence thesecondary coil voltage (measured by voltmeter 38 in FIGS. 5A and 5B) arekept independent of shaft properties regardless of what those particularproperties may be. The user never has to concern himself or herself withdetermining the RPM correction for each particular shaft and applyingthat correction to the output signal. Nor does recalibration have to bedone as shaft properties change over time, since the circuitry ofembodiment one automatically takes these changes into account, i.e., itautomatically eliminates spurious signal components regardless of theirnature or degree.

Embodiment two has the same advantages as embodiment one. The twosignals entering signal divider 56 are both affected by shaft propertiesin the same way, regardless of what those properties may be in an givenshaft. Hence the signal read by voltmeter 38 will not depend onvariations from shaft to shaft or on changing of shaft properties overtime.

For similar reasons embodiment four has the same advantages found inembodiments one and two. Division of two signals dependent in the samemanner on shaft properties results in a signal which is independent ofthe shaft properties.

Embodiment five is also amenable to mass production because it solvesthe misalignment and bending stress problems in a manner which isindependent of individual shaft bending or misalignment. Changes inalignment and bending stress over time will also automatically beaccounted for.

Advantages Over Prior Art

This invention can thus be seen to solve virtually all of the problemdelineated in the "Description of Prior Art" section presentlyassociated with determination of torque in rotating shafts in a muchmore complete, accurate, simple, economic, and straightforward mannerthan any previous art.

While the above description contains many specificities, the readershould not construe these as limitations on the scope of the invention,but merely as exemplifications of preferred embodiments thereof. Thoseskilled in the art will envision many other possibilities that arewithin its scope. For example any of the embodiments can use anymaterials, including supeconductors, for any components, and can haveany dimensions or shapes. The air gaps can be of any dimensions and caneven be of non-constant or non-uniform gap width. The shaft 20 can berotating or stationary and of any suitable material, size, or shape. Theshaft does not even have to be cylindrical and can have one or morestrips of material attached to it which enhance the working of theinvention. The strips can be of any suitable material including thinfilms and can be attached to any amount of the shaft.

Additionally, any number of coil windings can be used for any of thecoils in any of the embodiments and the cores can have any suitableshape and size as well as be of any suitable material. The cores do nothave to be ferromagnetic and can, if for instance the wires aresupeconducting, be made of air, any gas, any other material, or even avacuum. Also, whereas AC sinusoidal current is probably the mostsuitable for the invention, any wave form current/voltage can be used,even DC or a pulse with appropriate integration/differentiation.Wherever constant amplitude of any signal is referred to, any nearlyconstant amplitude signal which results in an acceptable measure oftorque will suffice. Further, any signal does not have to be useddirectly but can be amplified or transformed in any manner and theresultant amplified or transformed signal can be used for the same orsimilar purpose as the original signal. Also, any signal (e.g. voltage)does not have to be measured directly, but can be determined indirectlyvia measurement of parameters which are related to such a signal(e.g.'s, measuring other voltages in a loop, measuring current whenimpedance is known, etc.) Of course, all arrangements where volmeterswith one side attached to ground measuring the non-grounded side of acircuit can equivalently be arranged where the same volmeters measurevoltage across the circuit with neither side grounded. The converse istrue as well. Although the terms voltmeter and ammeter are used, anymeans to measure or determine voltage and amperage can be employed; andalthough terms such as oscillator and power amplifier are used, anydevices which serve the same or similar purposes for them or any othersystem components can be employed as well. Further, the sensors can belocated anywhere along the shaft axially, radially, or at the ends, inany number, and can even be used with a torque disk such as thatdescribed in U.S. Pat. No. 4,697,460 or other such appendage to theshaft. Also, any of the embodiments can be used separately or incombination in any degree with any of the others. Finally, the inventionin any of its embodiments can also be used as a device to stress andstrain, as in U.S. Pat. No. 2,912,642, and can even be used to measureforce applied to any given object or objects. Accordingly, the scope ofthe invention should be determined not by the embodiments illustrated,but by the appended claims and their legal equivalents.

We claim:
 1. A method for sensing torque comprising:inducing a primarymagnetic induction field flux in a torque transmitting element,obtaining a signal which depends on a secondary magnetic induction fieldflux in said torque transmitting element whose secondary magnetic fielddirection arises at a nonzero angle from the primary magnetic field as aresult of magnetostriction when torque is transmitted by the torquetransmitting element, and maintaining the primary magnetic inductionfield flux at effectively constant amplitude, whereby the secondarymagnetic induction field flux is affected minimally by non-torqueinduced variations in magnetic permeability in the torque transmittingelement and the signal which depends on the secondary magnetic inductionfield flux is thereby used to determine the torque transmitted.
 2. Themethod for sensing torque of claim 1 wherein the primary magneticinduction field flux is induced by at least one primary coil of anynumber of turns and maintaining said primary magnetic induction fieldflux at effectively constant amplitude comprises:using associatedelectric circuitry to control electric current into at least one primarycoil such that voltage across at least one auxiliary coil through whicha significant amount of the primary magnetic induction field flux passesis maintained at effectively constant amplitude.
 3. The method forsensing torque of claim 1 wherein obtaining a signal which depends onthe secondary magnetic induction field flux comprises:utilizing at leastone secondary coil of any number of turns and associatd secondaryelectric circuitry and determining a secondary circuitry parameter suchas, but not limited to, a secondary voltage across at least one saidsecondary coil, whereby the secondary circuitry parameter depends on thesecondary magnetic induction field flux and the secondary circuitryparameter is used to determine the transmitted torque.
 4. The method forsensing torque of claim 1 wherein the primary magnetic induction fieldflux is induced by at least one primary coil of any number of turns andmaintaining said primary magnetic induction field flux at effectivelyconstant amplitude comprises:using associted electric circuitry tocontrol electric circuit into at least one primary coil such thatvoltage across at least one auxiliary coil through which a significantamount of the primary magnetic induction field flux passes is maintainedat effectively constant amplitude; andobtaining a signal which dependson the secondary magnetic induction field flux comprises: utilizing atleast one secondary coil of any number of turns and associated secondaryelectric circuitry, and determining a secondary circuitry parameter suchas, but not limited to, a secondary voltage across at least one saidsecondary coil, whereby said secondary circuitry parameter depends onthe secondary magnetic induction field flux and said secondary circuitryparameter is used to determine the transmitted torque.
 5. The method forsensing torque of claim 1 wherein the primary magnetic induction fieldflux is induced by primary electric circuitry and at least one primarycoil of any number of turns and of such low electrical resistance thatany method for determining a primary circuitry parameter such as, butnot limited to, primary voltage across at least one said primary coil isa sufficiently accurate approximation to and a substitute for any methodfor determining an auxiliary circuitry parameter such as, but notlimited to, voltage across at least one auxiliary coil through which asignificant amount of the primary magnetic induction field flux passessuch that said primary circuitry parameter is controlled, therebyenabling the primary magnetic induction field flux to be maintained atsufficiently accurate approximate constant amplitude.
 6. The method forsensing torque of claim 5 wherein the means for obtaining a signal whichdepends on the secondary magnetic induction field fluxcomprises:utilizing at least one secondary coil of any number of turnsand associated secondary electric circuitry and determining a secondarycircuit parameter such as, but not limited to, voltage across at leastone said secondary coil, whereby said secondary circuitry parameterdepends on the secondary magnetic inductin field flux and said secondarycircuitry parameter is used to determine the transmitted torque.
 7. Aplurality of individual torque sensing methods, each of which is such asthe torque sensing method of claim 1, which comprises a misalignmentsignal correcting method for sensing torque whereinthe individual torquesensing methods are used in various locations around the torquetransmitting element such that angular spacings between said locationsare effectively equal, and wherein the individual torque sensing outputsignals of each of said plurality of individual torque sensing methodsare added to obtain a resultant signal, whereby said resultant signal isused to determine torque, and said resultant signal is minimallyaffected by bending stress and misalignment of the torque transmittingelement.
 8. A plurality of individual torque sensing methods, each ofwhich is such as the torque sensing method of claim 1, which comprises amisalignment signal correcting method for sensing torque whereintheindividual torque sensing methods are used in various locations aroundthe torque transmitting element such that angular spacings between saidlocations are not equal, and wherein the individual torque sensingoutput signals of each of said plurality of individual torque sensingmethods are phase shifted and then added to obtain a resultant signal,whereby said resultant signal is used to determine torque, and saidresultant signal is minimally affected by bending stress andmisalignment of the torque transmitting element.
 9. The method forsensing torque of claim 1 wherein maintaining the primary magneticinduction field flux at effectively constant amplitude comprises:usingassociated electric circuitry for passive control of the primarymagnetic induction field flux such that the primary magnetic inductionfield flux is maintained at effectively constant amplitude.
 10. Anapparatus for sensing torque comprising:a means for inducing a primarymagnetic induction field flux in a torque transmitting element, a meansfor obtaining a signal which depends on a secondary magnetic inductinfield flux in said torque transmitting element whose secondary magneticfield direction arises at a nonzero angle from the primary magneticfield as a result of magnetostriction when torque is transmitted by thetorque transmitting element, and a means for maintaining the primarymagnetic induction field flux at effectively constant amplitude, wherebythe secondary magnetic induction field flux is affected minimally bynon-torque induced variations in magnetic permeability in the torquetransmitting element and the signal which depends on the secondarymagnetic inductin field flux is thereby used to determine the torquetransmitted.
 11. The apparatus for sensing torque of claim 10 whereinthe primary magnetic induction field flux is induced by at least oneprimary coil of any number of turns and the means for maintaining saidprimary magnetic induction field flux at effectively constant amplitudecomprises associated electric circuitry comprising:a means forcontrolling electric current into at least one said primary coil suchthat voltage across at least one auxiliary coil through which asignificant amount of the primary magnetic induction field flux passesis maintained at effectively constant amplitude.
 12. The apparatus forsensing torque of claim 10 wherein the means for obtaining a signalwhich depends on the secndary magnetic induction field flux comprises atleast one secondary coil of any number of turns and associated secondaryelectric circuitry comprising:a means to determine a secondary circuitryparameter such as, but not limited to, a secondary voltage across atleast one secondary coil, whereby said secondary circuitry parameterdepends on the secondary magnetic induction field flux and saidsecondary circuitry parameter is used to determine the transmittedtorque.
 13. The apparatus for sensing torque of claim 10 wherein theprimary magnetic induction field flux is induced by at least one primarycoil of any number of turns, and the means for maintaining said primarymagnetic induction field flux at effectively constant amplitudecomprises associated electric circuitry comprising:a means forcontrolling electric current into at least one said primary coil suchthat voltage across at least one auxiliary coil through which asignificant amount of primary magnetic inductin field flux passes ismaintained at effectively constant amplitude; and the means forobtaining a signal which depends on the secondary magnetic inductionfield flux comprises at least one secondary coil of any number of turnsand associated secondary electric circuitry comprising: a means todetermine a secondary circuitry parameter such as, but not limited to, asecondary voltage across at least one said secondary coil, whereby saidcircuitry parameter depends on the secondary magnetic induction fieldflux and said secondary circuitry parameter is used to determine thetransmitted torque.
 14. A plurality of individual torque sensingapparata, each of which is such as the torque sensing apparatus of claim13, comprising a misalignment signal correcting apparatus for sensingtorque whereinthe individual torque sensing apparata are situated aroundthe torque transmitting element such that angular spacings between saidindividual torque sensing apparata are effectively equal, wherein theindividual torque sensing output signals of each of said plurality ofindividual torque sensing apparata are added to obtain a resultantsignal, whereby said resultant signal is used to determine torque, andsaid resultant signal is minimally affected by bending stress andmisalignment of the torque transmitting element.
 15. A plurality ofindividual torque sensing apparata, each of which is such as the torquesensing apparatus of claim 13, comprising a misalignment signalcorrecting apparatus for sensing torque whereinthe individual torquesensing apparata are situated around the torque transmitting elementsuch that angular spacings between said individual torque sensingapparata are not equal, wherein the individual torque sensing outputsignals of each of said plurality of individual torque sensing apparataare phase shifted and then added to obtain a resultant signal, wherebysaid resultant signal is used to determine torque, and said resultantsignal is minimally affected by bending stress and misalignment of thetorque transmitting element.
 16. The apparatus for sensing torque ofclaim 10 wherein the primary magnetic induction field flux is induced byprimary electric circuitry and at least one primary coil of any numberof turns and of such low electrical resistance that any means fordetermining a primary circuitry parameter such as, but not limited to,primary voltage across at least one said primary coil is a sufficientlyaccurate approximation to and a substitute for any means for determiningan auxiliary circuitry parameter such as, but not limited to, voltageacross at least one auxiliary coil through which a significant amount ofthe primary magnetic induction field flux passes such that said primarycircuitry parameter is controlled, thereby enabling the primary magneticinduction field flux to be maintained at sufficiently accurateapproximate constant amplitude.
 17. The apparatus for sensing torque ofclaim 16 wherein the means for obtaining a signal which depends on thesecondary magnetic induction field flux comprises at least one secondarycoil of any number of turns and associated secondary electric circuitrycomprising:a means to determine a secondary circuit parameter such as,but not limited to, voltage across at least one said secondary coil,whereby said secondary circuitry parameter depends on the secondarymagnetic induction field flux and said secondary circuitry parameter isused to determine the transmitted torque.
 18. A plurality of individualtorque sensing apparata, each of which is such as the torque sensingapparatus of claim 10, comprising a misalignment signal correctingapparatus for sensing torque whereinthe individual torque sensingapparata are situated around the torque transmitting element such thatangular spacings between said individual torque sensing apparata areeffectively equal, wherein the individual torque sensing output signalsof each of said plurality of individual torque sensing apparata areadded to obtain a resultant signal, whereby said resultant signal isused to determine torque, and said resultant signal is minimallyaffected by bending stress and misalignment of the torque transmittingelement.
 19. A plurality of individual torque sensing apparata, each ofwhich is such as the torque sensing apparatus of claim 10, comprisingalignment signal correcting apparatus for sensing torque whereintheindividual torque sensing apparata are situated around the torquetransmitting element such that angular spacings between said individualtorque sensing apparata are not equal, wherein the individual torquesensing output signals of each of said plurality of individual torquesensing apparata are phase shifted and then added to obtain a resultantsignal, whereby said resultant signal is used to determine torque, andsaid resultant signal is minimally affected by bending stress andmisalignment of the torque transmitting element.
 20. The apparatus forsensing torque of claim 10 wherein the means for maintaining the primarymagnetic induction field flux at effectively constant amplitudecomprises associated electric circuitry comprising:a means for passivecontrol of the primary magnetic induction field flux such that theprimary magnetic induction field flux is effectively constant amplitude.21. A method for sensing torque comprising:inducing a primary magneticinduction field flux in a torque transmitting element, the primarymagnetic induction field flux being induced by at least one primary coilof any number of turns, obtaining a signal which depends on a secondarymagnetic induction field flux in said torque transmitting element whosesecondary magnetic field direction arises at a nonzero zngle from theprimary magnetic field as a result of magnetostriction when torque istransmitted by the torque transmitting element, and controlling currentinto the least one primary coil such that the voltage across at leastone auxiliary coil through which a significant amount of primarymagnetic induction field flux passes is maintained at effectivelyconstant amplitude, whereby the secondary magnetic induction field fluxis affected minimally by non-torque induced variations in magneticpermeability in the torque transmitting element and the signal whichdepends on the secondary magnetic induction field flux is thereby usedto determine the torque transmitted.